Polyester films, polyethylene terephthalate films in particular, having excellent mechanical characteristics, thermal characteristics, and electrical characteristics, are widely used in various industrial fields and are in increasing demand. However, as the applications broaden and the demands increase, there is a greater need for improvements in the characteristics and productivity of polyesters. Therefore, although polyester films are produced for various purposes, such as for industrial use and for magnetic material use, there are many problems to be solved.
In general, when a polyester film is formed, a polymer produced by polymerization is melted again to form the polyester film. There is residence time in the process in which the polymer is melted again and melt extrusion is performed. The polymer deteriorates during the residence time, resulting in an increase in filter pressure due to clogging of the filter and defects in the film product, thus giving rise to problems. Pyrolysis, oxidative degradation, hydrolysis, etc., of the polymer are considered to be the reasons for the above, and it is supposed that a metal catalyst present in the system promotes the oxidative degradation and hydrolysis of the polymer.
In order to improve productivity in the film-forming process, desirably, a film is electrostatically charged and brought into close contact with a casting drum when cooling and setting are performed. In order to perform casting by the electrostatic casting method, a metal at a predetermined amount or more is desirably added to the polymer. However, although productivity is improved by the addition of the metal, heat resistance is degraded because the metal acts as a degradation catalyst, which is undesirable. It is known that, in order to improve heat resistance, preferably, the metal additive is inactivated or a portion of the metal is removed. However, with respect to a currently known method in which metal additive is inactivated using a phosphorus compound, although the metal can be inactivated, the electrostatic adhesion between the molten film and the casting drum is decreased, which is problematic.
Furthermore, Japanese Unexamined Patent Application Publication No. 2000-34343 discloses a method for recovering and removing a catalyst from ethylene glycol produced in a polycondensation step. However, no proposition has been made on recovery and removal of a metal catalyst from a polyester composition.
As described above, there are no known methods by which it is possible to remove or inactivate a metal catalyst, which adversely affects heat resistance, without decreasing electrostatic adhesion between a molten film and a casting drum.